In conventional automotive braking systems, a master cylinder converts driver exerted brake pedal force into a corresponding hydraulic pressure, which is proportioned among the front and rear brakes. In power assisted braking systems, a vacuum booster is interposed between the pedal and the master cylinder to amplify the force applied to the master cylinder by using engine vacuum to create a pressure differential across one or more diaphragms coupled to the master cylinder.
As an alternative to the above-described conventional braking system, it has been proposed to use electrically powered boost units to develop the amplified brake pressures. Such systems--referred to herein as electro-hydraulic, or EH, systems--can advantageously be used, for example, in electric vehicles where there is no convenient vacuum source. Even in vehicles powered by an internal combustion engine, EH braking systems can be used to advantage for integrating advanced braking controls such as anti-lock braking and traction control.
One example of an EH braking system is disclosed in the U.S. Pat. No. 5,362,135 to Riddiford et al., issued Nov. 8, 1994, assigned to the assignee of the present invention, and incorporated herein by reference. In that system, the master cylinder pressure is coupled through normally open solenoid operated fluid valves and electrically powered boost units to the front service brakes of an electric vehicle. The master cylinder brake pressure developed in response to driver exerted brake pedal force is measured and used to develop a brake pressure command for the electrically powered boost units. The pressures actually developed by the boost units are measured to provide feedback to the controller, and the controller activates the boost units as required to bring the feedback signals into agreement with the pressure commands.
One aspect of the above-described EH braking control is that the control gains for the boost units are preferably temperature dependent to a certain degree. In particular, at cold ambient temperatures the hydraulic fluid has increased viscosity. While a temperature sensor may be used for gain scheduling, such sensors are sometimes not available in lower cost systems. Accordingly, what is desirable is a cold temperature compensation control which does not require the use of a temperature sensor.